In what way are [1] not “full OSes”? They’re minimal templates, but afaik they still run systemd, the kernel, etc. needed to boot the standard Linux systems they are.
When I clicked the link I was expecting something like a unikernel, eg https://roscidus.com/blog/blog/2016/01/01/a-unikernel-firewa...
You certainly can run distros without systemd [0] or something very different like *BSD or Mirage [2].
[0] https://forum.qubes-os.org/t/devuan-or-other-non-systemd-tem...
https://forum.qubes-os.org/t/alpine-linux-template-non-offic...
[1] https://forum.qubes-os.org/t/openbsd-as-a-ubes-os-component/...
[2] https://forum.qubes-os.org/t/mirage-firewall-0-9-0-released/...